The present invention relates to a vertical-cavity surface-emitting laser (VCSEL) device comprising a plurality of VCSEL elements arranged on a common substrate, each VCSEL element comprising first mirror means having a first reflectivity and second mirror means having a second reflectivity at a predetermined wavelength for forming an optical resonator for said wavelength, and a laser active region disposed between said first and second mirror means.
Semiconductor laser devices steadily gain in importance in a plurality of industrial applications. In particular, in the fields of gas spectroscopy, coupling of laser light into optical fibers, and in communication systems where a high transmission rate is required, semiconductor laser devices with high spectral purity, i.e. with single mode output radiation in the longitudinal as well as the transverse directions, are highly desirable. Especially, vertical-cavity surface-emitting lasers (VCSEL) represent an important development, since the possibility of manufacturing a large plurality of such laser devices on a single semiconductor substrate provides laser devices with high efficiency and low power consumption in conjunction with low manufacturing costs. These laser devices inherently lase in a single longitudinal mode due to their small longitudinal extension of the laser active region (some hundreds of nanometers). When, however, a high output power from a single device is required, the lateral extension of this device has to be increased, thereby reducing the spectral purity of the laser output, since then the beam quality suffers from the competition of many transverse radiation modes. Accordingly, the highest possible single mode output power from a VCSEL is limited (currently the maximum value achieved is 4.8 mW), since the size of the VCSEL must remain small to restrict emission to a single fundamental transverse mode.
In order to achieve increased output power while maintaining a well-defined single transverse mode which is desirable for a variety of applications such as laser printing, material treatment, or optical pumping, so-called xe2x80x9cphase coupled arraysxe2x80x9d have been developed and investigated during the last years. In such a phase coupled array, usually the top or bottom surface of a laterally widely extending VCSEL is divided into a plurality of laser elements by means of a grid-like structure, typically formed of metal. The thickness of the grid bars separating adjacent laser elements are selected so as to allow the electric fields of adjacent elements to couple to each other. Since, in general, top and bottom distributed Bragg reflectors as well as the laser active region are provided common to all single laser elements, and a current is supplied to the common active region by means of the conductive grid bars, the laser elements are no longer individually addressable. Accordingly, the phase coupled array can also be considered a laterally large VCSEL device emitting in a coherent supermode, wherein the nodes of the electric field are defined by the grid structure on the top or bottom surface of the VCSEL.
In the early 1990""s, phase coupled arrays were demonstrated for the first time and, in recent developments, have shown very promising behavior in pulsed operation with more than 500 mW of a single mode peak output power in, for example, an 8xc3x978 array, as disclosed in xe2x80x9cApplied Physical Lettersxe2x80x9d Vol. 61, 1160 (1992).
In order to provide for a phase coupled array in a VCSEL, a variety of possibilities have been practiced in the prior art.
In IEEE xe2x80x9cJournal of Quantum Electronics,xe2x80x9d Vol. 26, No.11, November 1990, a phase coupled array is described having a metal grid inside the cavity defining areas of low reflectivity. Subsequently, a dielectric mirror has been deposited after the formation of the metal grid and this dielectric and serves as the outcoupling mirror of the laser device. This fabrication technology, however, is quite complicated and the device exhibits during operation a mixture of the lowest order and several high order modes, so that this approach does not seem to be very promising.
In xe2x80x9cOptics Letters,xe2x80x9d Vol. 18, No.5, Mar. 1, 1993, a VCSEL is disclosed having a metal grid applied to the top of a complete VCSEL structure, including two semiconductor distributed Bragg reflectors. However, the reflectivity variation induced by the metal grid alone, is too low to stabilize the highest order transverse mode for CW operation.
In xe2x80x9cApplied Physical Letters,xe2x80x9d Vol. 60, 1535, 1992, a bottom emitting VCSEL structure is described, having a semiconductor bottom mirror and a hybrid semiconductor/gold top mirror. The reflectivity of the top mirror is fine-tuned with two different metalizations, wherein highly reflective gold is evaporated on the laser elements, while less reflective TiAu or Cr/Au is used for the grid which defines the individual laser elements. Since no light can escape through the top metals, this technology is only appropriate for bottom emitters.
In xe2x80x9cApplied Physical Letters,xe2x80x9d Vol. 58, 890, 1991,a VCSEL is disclosed, wherein a grid is etched into the top distributed Bragg reflector. According to this technology, no current injection is possible and, hence, the device is merely able to be operated with optical pumping.
It is, therefore, an object of the present invention to provide a VCSEL device having a high output power with a defined single transverse radiation mode, whereby the aforementioned disadvantages of the prior art are avoided.
According to the present invention, there is provided a vertical-cavity surface-emitting laser device comprising a plurality of VCSEL elements arranged on a common substrate, each VCSEL element comprising first mirror means and second mirror means each having a predefined reflectivity at a predetermined wavelength, for forming an optical resonator for said wavelength, a laser active region disposed between said first and second mirror means, and a grid layer arranged over said first mirror means, said grid layer having a plurality of openings corresponding to the respective VCSEL elements, said VCSEL device being characterized in that it comprises a contact layer having a predetermined thickness, said contact layer being interposed between and adjacent to each of said first mirror means and said grid layer, wherein an optical thickness of said contact layer and a reflectivity and an absorption of said grid layer is selected so as to provide an effective reflectivity of each of said first mirror means depending on said grid layer and being different for areas covered by the grid and areas corresponding to said grid openings.
According to the present invention, the employment of the contact layer in combination with the structure of the grid layer, i.e. the geometric structure as well as the composition and the thickness thereof, provides simple means for a significant variation of the reflectivity of the first mirror means. Due to the varying reflectivity along the lateral dimension of the VCSEL device, the loss within the cavity varies accordingly and, hence, a single transverse radiation mode is sufficiently stabilized. Therefore, the VCSEL device of the present invention allows, contrary to the prior art devices which are described to be operated only in a pulsed mode, the operation with constant current and continuous wave (cw) thereby insuringe high continuous output power exhibiting a single transverse radiation mode.
Preferably, said first and second mirror means are provided as Bragg reflectors which are common to all of said VCSEL elements. By this measure, a high density of VCSEL elements may be provided, and, at the same time, the manufacturing process for such a device is considerably simplified. Moreover, manufacturing of such devices is compatible to standard fabrication methods such as selective oxidation, mesa etching, and proton implantation.
Advantageously, the thickness of said contact layer is adjusted such that the reflectivity of said first mirror means is reduced in regions covered by the grid, compared to regions corresponding to said grid openings. An accordingly arranged contact layer and grid layer insure that the reflectivity and, hence, the loss of the resonator in regions corresponding to the portions covered by the grid, is relatively low so that the transverse radiation mode having an intensity minimum at those portions; is significantly stabilized, whereas other transverse radiation modes are remarkably suppressed.
In a preferred embodiment, the thickness of said contact layer is adjusted so as to obtain the minimum reflectivity of said regions covered by said grid layer for a given composition of the first mirror means and said grid layer.
By this measure, the lateral reflectivity contrast reaches a maximum value and, hence, the suppression of undesired transverse radiation modes is maximized.
Preferably, said grid layer comprises more than one layer, each of said layers consisting of a material having a defined index of refraction, a defined absorption coefficient and a defined thickness.
Advantageously, said regions not covered by said grid layer form radiation emission windows for a radiation of said wavelength. In this way, the radiation is output through the opening of the grid layer and the bars of the grid layer additionally serve as an optical aperture.
Preferably, an extension of each of said radiation emission windows is arranged to select substantially a fundamental transverse radiation mode of said radiation with respect to the corresponding VCSEL element. This leads to an arrangement in which the VCSEL device in its entirety emits in the highest order transverse radiation mode (supermode).
Advantageously, the absolute reflectivity of said first and second mirror means including optical characteristics of said contact layer and said grid layer is selected so as to obtain the minimum reflectivity required to generate stimulated emission. Thus, it is guaranteed that the VCSEL device emits a laser beam once a certain current is supplied to the active region.
Preferably, said grid layer comprises at least one metal layer. This allows to use the grid layer concurrently as an electrode for injecting charge carriers into the active region.
Advantageously, an additional dielectric mirror is provided on top of said first mirror means so as to further increase the overall reflectivity of the resonator.
Preferably, said first reflectivity, in the areas covered by the grid, is reduced by at least 3 percent compared to the reflectivity of said areas corresponding to the grid openings.
In a preferred embodiment, said grid layer is a raster of squares or rectangles. In such an arrangement, each pixel can naturally couple to its nearest neighbors 180xc2x0 out of phase, which leads to emission in the highest order supermode of the device.
Advantageously, a bar width of said grid layer is less than 10 xcexcm and is preferably less than 5 xcexcm (typically 1 xcexcm). An accordingly arranged bar width of said grid layer separating adjacent radiation emission windows allows the electric fields of corresponding adjacent VCSEL elements to sufficiently couple to each other so that a single transverse radiation supermode is maintained. However, with thinner bars, not only the coupling between adjacent VCSELs is stronger but the area coverage of the bars is smaller, thereby making the coupling of the light out of the device more efficient.
Preferably, a maximum dimension of each radiation emission window of each VCSEL element is in the range of 2 xcexcm to 10 xcexcm. With these dimensions, the fundamental mode of each VCSEL element is efficiently selected.
In a preferred embodiment, the grid layer is a honeycomb raster and a resistance layer is provided, formed between the first and second mirror means and having at least one region corresponding to an opening of said honeycomb raster with a high electrical resistance and said at least one high resistance region has 6 nearest-neighbor-regions having a low electrical resistance.
Since the highest order supermode requires two adjacent VCSEL elements to emit radiation with a phase difference of 180xc2x0, each VCSEL element in the center of 6 surrounding elements in the honeycomb raster does not meet this requirement. By providing said resistance layer which at least covers the area of a central element, the charge carriers flowing through the active region are forced to concentrate in the neighboring VCSEL elements having low electrical resistance. Therefore, although the net current flow remains at the same level, the charge carrier density in the active regions of the VCSEL elements neighboring said central element is increased, thereby increasing the conversion efficiency of these active regions.
Preferably, said high resistance region comprises an exposed portion where at least the overlying layers of the Bragg mirror have a vertical opening. By means of this vertical opening, the high resistance region can easily be manufactured by selective oxidation of a buried layer having a high content of aluminum.
In a further preferred embodiment, a grid matching resistance layer is provided between said one of said mirror means and said laser active region, said grid matching resistance layer having a high electrical resistance at least in regions covered by said grid layer. Since the geometric structure of the grid matching resistance layer matches the geometry of said grid layer, the charge carrier density can be increased in regions where the reflectivity is high and, thus, the optical gain is additionally improved.